According to the prior art, parts made from wood or woody materials are interconnected, e.g., using joining elements in the form of nails or screws which are driven through one of the parts to be joined into the other part to be joined. Screws and nails are generally made from metal and have a head in a surface area of one of the parts to be joined and are at least frictionally or positively anchored in another of the parts to be joined. These pin-like joining elements constitute in wooden structures metallic, often corrosive, foreign bodies which can be prejudicial to working after the joining of the parts and which represent heat transfer bridges in the finished structures.
It is also known to join parts made from fibrous materials, including wood and wood-like materials, to parts made from thermoplastics. The thermoplastic is plasticized at its surface facing the fibrous material part and the two surfaces are pressed onto one another. The plastic is thereby applied in plasticized form to the surface of the fibrous material or it is plasticized in the final position, e.g., by ultrasonic excitation. In all of these methods, a joint is formed at the interface between the plastic and the fibrous material in the sense of a microscopic interlocking in that the plasticized plastics material is pressed into surface irregularities of the fibrous material. Such methods are, e.g., described in FR-2 455 502, FR 1 495 999, DE-38 28 340 or EP-269476. According to WO-96/01 377, the plastics part can also be a dowel, which joins together two wooden parts. The same principle forms the basis for known methods for joining parts made from wood or wood-like materials, in which between the parts to be joined is placed a layer of a thermoplastics material, e.g., a paint layer, and the parts are then pressed together and subjected to ultrasonic action (JP 52 127 937, WO 96/01377).
In all the aforementioned methods, synthetic material and fibrous material are joined together by surface adhesion resulting from microscopic interlocking and this occurs in the same way in conventional bonding or adhesion processes. The above methods have many of the same disadvantages as bonding methods, particularly their sensitivity to moisture and thermal stresses in which the two joined-together surfaces expand to varying degrees and considerable shear forces arise which weaken or even destroy the joint.